Sprint Nextel is ending unlimited data plans for all devices except smartphones, bringing the era of all-you-can-eat mobile data in the U.S. nearer to a close.

In a notice posted Thursday on the customer support section of its website, Sprint said it would impose monthly data caps on plans for all tablets, laptops, netbooks, USB and PC Card modems, and mobile hotspot devices. Data will also be capped for Mobile Hotspot plans that let subscribers connect other devices to the network through a smartphone. The caps will begin with each subscriber's next bill following notification, the carrier said.

However, Sprint will continue to offer its plans for unlimited data use on phones, including on the Apple iPhone, which Sprint introduced just last week.

The change eliminates one of the key differentiators Sprint had used to market its services against larger rivals Verizon Wireless and AT&T. Those carriers had already capped their data plans, and the nation's fourth-largest operator, T-Mobile USA, announced in April that it would throttle the connection speed of customers who exceeded 2GB of data usage per month.

While capping data use on its 3G network to 3GB, 5GB or 10GB per month, Sprint has continued to offer unlimited 4G service. Now, the cap for each of these plans will cover the subscriber's combined monthly data use on both 3G and 4G. All data use above the cap will cost $0.05 per megabyte.

The monthly allowance for data use while roaming on non-Sprint networks will be 100MB for the 3GB plan and 300MB for the other two plans, with each megabyte over those caps charged at $0.25.

For Mobile Hotspot plans on phones, data usage will be capped at 5GB per month of either 3G or combined 3G/4G service, depending on whether the phone can use 4G. Use above that cap will be charged at $0.05 per megabyte. Those plans will have a 300MB monthly cap for data roaming.

Networks built with 4G technology, such as the Clearwire WiMax network that Sprint uses for its 4G service, are more efficient than 3G networks. Because they can deliver more data over a given amount of radio spectrum, they can make it more economical for a carrier to offer a certain amount of data per month or an unlimited plan. In addition, because 4G networks are fairly new, they are less likely to suffer from the overloading problems that some carriers have cited when imposing monthly limits.

However, U.S. carriers have been moving away from unlimited plans as the amount of data flowing over mobile networks has grown dramatically and showed no signs of slowing down. A recent survey by investment bank Credit Suisse showed that mobile networks in the U.S. were running at an average of 80 percent of capacity.

Sprint resells 4G service from Clearwire under an agreement reached earlier this year, in which it will pay the WiMax carrier about $1 billion through the end of next year. But it also plans to build its own 4G LTE (Long-Term Evolution) network and launch it commercially in the middle of next year.